Electrolytic removal of metallic coatings from various base metals



Patented Dec. 18, 1951 ELECTROLYTIC REMOVAL OF METALLIC COATINGS FROM VARIOUS BASE METALS Henry G. Orlik, Camden, N. J assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application August 20, 1948, Serial No. 45,423

11 Claims. 1

This invention refers, generally, to an improved process for removing coatings of any one of several metals from a number of different types of underlying metals. More particularly, the invention relates to a process which utilizes an improved stripping bath.

In general, the process comprises immersing the article which is to have its surface coating removed, in the improved bath of the present invention, connecting up the article as an anode and, preferably utilizing copper or lead cathodes, passing a current of electricity through the solution.

The principal object of the present invention is to provide an improved process of stripping many diiierent metals from various underlying metals without etching the underlying metal.

Another object of the present invention is to provide an improved process for removing metal coatings from various underlying metals in a more rapid and eiiicient manner than can be done with previous processes.

Another object of the present invention is to remove nickel, copper, chromium, zinc, cadmium, tin or silver from any of steel, brass, zinc-bas castings or aluminum.

These and other objects will be more apparent and the invention will be better understood from the following specification.

A preferred embodiment of the bath which constitutes a necessary part of the improvement of the present invention should preferably comprise by weight about 64 to about 90 percent orthophosphoric acid, about to about 14 percent acetic and about 5110 about 24 percent water. Although not necessary to accomplish good results, glycerine may be added to this bath, also, especially for preventing attack on steel. The specific gravity of the bath will preferably range between 1.5 and 1.65.

Example I A bath was made up comprising '70 percent by weight of orthophosphoric acid, 6.6 percent by weight of glacial acetic acid and 23.4 percent by weight of water. Copper cathodes were used while the work was connected as anode. The bath was operated at a current density of between 50 and 150 amps. per square foot and at a temperature slightly above room temperature. The cathode area was 2 to 4 times the anode area. Electrodeposited coatings of chromium, copper and nickel were removed from steel and also from brass. The condition of the steel at the end of the run was excellent and only very slight etching was noticed on the brass. Removal of the deposit was rapid and complete.

Example II A bath similar to that of Example I was made up except that the amount of orthophosphoric acid was 64.7 percent by weight, 13.6 percent by weight glacial acetic acid and 21.7 percent by weight of water. This bath was excellent for removing any of copper, nickel, chromium, zinc, cadmium, tin or silver from iron or steel and was also very efiicient in the removal of any of these metals from brass. Current densities and other conditions were the same as in Example I.

Erample III It has also been found that concentrated sulfuric acid percent H2804) can be substituted for up to about half the volume of glacial acetic acid present in any of the baths described above and the resulting solution can then be used to rapidly remove copper, nickel, chromium, zinc, cadmium, tin or silver from any of steel, brass, zinc base castings or aluminum. Calculated on a weight basis, all ingredients being of percent concentration, the ranges of preferred percentages then are orthophosphoric acid, about 61 to about 86 percent, sulfuric acid, about 5 to about 12 percent, acetic acid, about 3 to about 6.5 percent, and water, about 6 to about 25 percent. The results are nearly as good as for the solutions described in Examples I and II.

Using an of the baths heretofore described, various changes may be made in operating conditions in the process, within the spirit of the present invention. For example, the bath works better after it has been broken in. The breaking in process consists of working the bath with parts to be stripped of metal so that there is a solution of the metal to be stripped in the electrolyte before the actual stripping operation commences. The same effect can be obtained, however, by introducing into the bath, directly, one or more soluble salts of the metal which is to be stripped, provided this salt has the same anion as either of the acids in the bath. Also, the anion of the salt may be a carbonate or any otheranion which decomposes to form harmless gases when the compound is broken down with an electric current. Thus, if copper is to be stripped in a bath containing acetic and orthophosphoric acids, the copper salt which is introduced may be either copper carbonate, copper 55 acetate or copper phosphate. If chromium is be stripped, a chromate the metal which is to salt may be used.

Although the bath is generally operated at room temperature, the temperature is by no means critical and any other temperature, such as may be used in general electroplating processes, may be jusedl j' As in electroplating processes, the higher the temperature the more rapid is the chemical reaction which takes place. Temperatures of 120 F. have actually been measured in the baths and temperatures far below room temperature have also been tried, 7, V

The bath is preferably operated at a potential of from 1 to 12 volts. If potentialsof from 9 to 12 volts are used, current densities'near the high end, that is, the upper limit, of the 50 to 150 amps. per square foot range can be .used with much more rapid removal of the plated deposits; Although acetic and orthophosphoricacids are preferred as principal ingredients in the bath,"

there may also be used salts of these acids with somewhat less satisfactory results, which vary with the cation which is used. In general, it has been found that alkali metal cations affect the results least.

Although the solutions'described in Examples I and II'are preferred for use with the metals listed therein, they may be used with almost equally satisfactory results with the additional metal coatings and bases listed in Example III.

There has thus been described an improved process for the more efiicient removal of many metals from various underlying metals. The process is dependent upon use of the improved solutions which-have been described. Compared with former processes using, for example, phosphoric acid alone, the present process is faster and does not attack brass as does phosphoric acid. Solutions comprising sulfuric and phosphoric acids only have also been compared with the solutions of the present invention but the former etch steel as the percentage of phosphoric acid is increased, exhibit some attack on brass at all concentrations tried and become increasingly slow if the relative percentage of sulfuric acid is raised.

What I claim is:

l. A method of stripping a coating metal of the class consisting of nickel, copper, chromium, zinc; cadmium, tin, and: silver from an underlying metal of the class consisting of brass, zinc base castings, aluminum, and steel which comprises making; the metal part to be stripped, the anode in a solution consisting essentially of about 64 to about 90 percent orthophosphoric acid, about to about 14 percent acetic acid, and about 5 to about 24 percent water, all percentages being by weight, and passing an electrical current through said solution until. a desired amount of said coating metal has been removed.

2. A method of stripping a coating metal of the class consisting of nickel, copper, chromium, zinc, cadmium, tin, and silver from an underlying metal of the class consisting of brass, zinc base castings, aluminum, and steel which com- 4 prises making the metal part to be stripped the anode in a solution consisting essentially of orthophosphoric acid about 61 to about 86 per cent, sulfuric acid about 5 to about 12 percent, acetic acid about 3 to about 6.5 percent, and water about 6 to about 25 percent, all percentages beingby. weight, and passing an electrical current through said'solution until a desired amount of said coating metal has been removed.

3. A method according to claim 1 in which the acid radicals are added in the form of their solublemetallic salts.

4. A method according to claim 1., including adding to said solution a soluble metallic salt of which the'cation is the same as the metal to be removed from the surface of the article.

5. A method, according to claim 1, in which said coating metal is copper and said underlying metal is steel.

6. A method, according to claim 1, in which said coating metal is nickel and said underlying metal is'brass.

7. A method, according to claim 1, in which said coating metal is chromium and said underlying metal is steel.

8. A method according to claim 2 in which the acid radicals are added in the form of their soluble metallic salts.

9. A method according to claim 2 including adding to said solution a soluble metallic salt of which the cation is the same as the metal to be removed from the surface of the article.

10. A bath for strippingmetal coatings from underlying metals by electrolytic anodic action, saidv bath consisting essentially of about 64 to about percent orthophosphoric acid, about 5 to. about14 percent acetic acid and about 5 to about 24 percent water, all percentages being by weight.

11. A bath for stripping metal coatings from underlying metals by electrolytic anodic action, said bath consisting essentially of about 61 to about 86 percent orthophosphoric acid, about 5 to about 12 percent sulfuric acid, about 3 to about 6.5 percent acetic acid, and about 6 to about 25 percent water, all percentages being by weight.

HENRY G. ORLIK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,528,144 Couesnon Mar. 3, 1925 1,867,527 Dunn July 12, 1932 2,436,244 Benner et al Feb. 17, 1948 FOREIGN PATENTS Number Country Date 707,526 France Apr. 14, 1931 OTHER REFERENCES Transactions of The Electrochemical Society, volume 69 (1936), page 629. 

1. A METHOD OF STRIPPING A COATING METAL OF THE CLASS CONSISTING OF NICKEL, COPPER, CHROMIUM, ZINC, CADMIUM, TIN, AND SILVER FROM AN UNDERLYING METAL OF THE CLASS CONSISTING OF BRASS, ZINC BASE CASTINGS, ALUMINUM, AND STEEL WHICH COMPRISES MAKING THE METAL PART TO BE STRIPPED THE ANODE IN A SOLUTION CONSISTING ESSENTIALLY OF ABOUT 64 TO ABOUT 90 PERCENT ORTHOPHOSPHORIC ACID, ABOUT 5 TO ABOUT 14 PERCENT ACETIC ACID, AND ABOUT 5 TO ABOUT 24 PERCENT WATER, ALL PERCENTAGES BEING BY WEIGHT, AND PASSING AN ELECTRICAL CURRENT THROUGH SAID SOLUTION UNTIL A DESIRED AMOUNT OF SAID COATING METAL HAS BEEN REMOVED. 